Superantigens (SAgs) for B lymphocytes interact via conserved V region framework sites in the B cell antigen receptor (BCR) to target large sets of lymphocytes. We have previously elucidated central structural and immunobiologic properties of protein A of Staphylococcus aureus (SpA), and established SpA as the prototypic experimental B-cell superantigen. Based on an understanding of the molecular basis by which naturally pentameric SpA binds B-cells, we have recently developed the murine T15i Ig "knockin" system for investigations of the in vivo outcome of SpA exposure. In these mice, most B cells express a VH transgene product that is targeted by SpA, and we have shown that SpA treatments rapidly induce activation-associated apoptotic death of targeted B cells. In the current research program, we will use different forms of SpA to elucidate key mechanisms responsible for BCR-mediated determinations of lymphocyte clonal fate. The Specific Aims will include: AIM 1: To define the nature of the SAg-induced BCR complex responsible for B lymphocyte activation and apoptosis. AIM 2: To determine how membrane co-receptors may affect clonal fate after interactions with SpA. AIM 3: To investigate how Bcl-2 family members may be involved in determining B-cell clonalfate after interactions with SpA. AIM 4: To evaluate how co-exposure to other immunologically active components of S. aureus can affect the outcome of in vivo SpA exposure. These investigations will provide important insights into the fundamental properties of B-cells. In addition, these studies will lead to a better understanding of the immunobiologic activities of a virulence factor from one of the most important causes of life-threatening infection in the US.